Remediation of Sensitization in Metals

ABSTRACT

An ultrasonic impact treatment method for remediating metal sensitization including introducing ultrasound compression wave energy through ultrasonic mechanical impulse impacts into an area of sensitized metal in a workpiece. The ultrasound compression wave energy and impulse impacts impart compressive residual stress to the workpiece thereby decreasing tensile stresses in the sensitized metal and modifying the grain structure of the workpiece. These changes to the structure of the workpiece combine to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, cause intergranular diffusion of alloying elements in the area of sensitized metal, return a portion of alloying elements in the area of sensitized metal to solution and reduce or eliminate substantially straight intergranular paths through the workpiece.

FIELD OF THE INVENTION

The present invention is directed to a method for remediating sensitization in metals, and more particularly, to remediating sensitization in metals by application of ultrasonic impact treatment (UIT).

BACKGROUND OF THE INVENTION

Sensitized metals are those that when exposed to high temperatures for extended periods have alloying phases precipitate to the grain boundaries of the metal. Precipitation of the alloying phases makes the materials very susceptible to cracking and material failure. Stress corrosion cracking (SCC) refers to the growth of a crack in a susceptible material that is subjected to tensile stress above a threshold value and exposed to either a gaseous or liquid corrosive environment. A non-exhaustive list of examples of materials susceptible to SCC include carbon steels, low alloy steels, high strength steels, all 300-series stainless steels (including Types 304, 304L, 304H, 321, and 347), aluminum alloys from the 5XXX alloy family which may be sensitized, copper alloys and titanium alloys. Examples of corrosive environments include but are not limited to, hydroxides, nitrates, carbonates, bicarbonates, liquid ammonia, carbon monoxide/carbon dioxide/water, aerated water, chloride, sulfide, thiosulfate, polythionate, hydrogen sulfide, and methanol. When cracking or material failure occurs in sensitized metal, wholesale removal and replacement of the sensitized metal are required since there is no effective repair technique.

SUMMARY OF THE INVENTION

The present invention is directed to the application of UIT to sensitized metals for effectively remediating the effects of metal sensitization or repairing the sensitized metals using conventional welding procedures. According to one aspect of the invention, there is provided a method for treating metal including providing a workpiece including an area of sensitized metal and decreasing tensile stresses in the area of sensitized metal by imparting compressive residual stress in the area of sensitized metal. Compressive residual stress is imparted to the workpiece by applying a multiplicity of shock pulses in the form of ultrasonic energy with an ultrasonic transducer to the area of sensitized metal thereby creating a treatment zone of plastic material in the metal structure. It is believed that the compressive residual stress imparted to the area of sensitized metal acts to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, causes intergranular diffusion of alloying elements in the area of sensitized metal, returns a portion of alloying elements in the area of sensitized metal to solution and reduces or eliminates substantially straight intergranular paths through the workpiece to a surface thereof.

According to another aspect the invention, there is provided a method for treating metal including providing a workpiece including an area of sensitized metal, the area of sensitized metal including a grain structure including a plurality of crystal grains, and modifying the grain structure by arranging a major axis of each grain of a portion of grains of the plurality of grains to be essentially parallel to a surface of the area of sensitized metal. The grain structure is modified as described above by applying a multiplicity of shock pulses to the area of sensitized metal in the form of ultrasonic energy with an ultrasonic transducer in contact with a surface of the workpiece. Modification of the grain structure in this manner is believed to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, cause intergranular diffusion of alloying elements in the area of sensitized metal, return a portion of alloying elements in the area of sensitized metal to solution and reduce or eliminates substantially straight intergranular paths through the workpiece to a surface thereof.

According to another aspect of the invention, there is provided a method for treating metal including providing a workpiece including an area of sensitized metal, replacing a portion of the area of sensitized metal with a replacement metal, and introducing ultrasound wave energy into the workpiece about a junction of the replacement metal with the workpiece. The ultrasound wave energy is introduced in the form of ultrasonic energy with an ultrasonic transducer in contact with a surface of the workpiece. The ultrasonic wave energy is believed to ultrasonically excite the base metal and relax stresses therein thereby making the base metal more susceptible to grain modification produced by the impact of a set of indenters coupled between the metal surface and the ultrasonic transducer. In this way, it is believed that the introduction of the ultrasound compression energy acts to slow the rate of enrichment of alloying elements at grain boundaries within the area of sensitized metal, causes intergranular diffusion of alloying elements in the area of sensitized metal, returns a portion of alloying elements in the area of sensitized metal to solution and reduces or eliminates substantially straight intergranular paths through the workpiece to a surface thereof.

According to another aspect of the invention, there is provided a workpiece including a sensitized metal portion having a treatment zone, the treatment zone being constructed and arranged by introducing pulses of ultrasonic wave energy into the sensitized metal portion through periodic ultrasonic mechanical impulse impacts. As a result of the introduction of the ultrasonic wave energy through ultrasonic mechanical impulse impacts, a grain structure of the sensitized metal portion, which includes a plurality of crystal grains, is modified so that each grain of a portion of grains of the plurality of grains has a major axis arranged essentially parallel to a surface of the treatment zone. It is believed this modification of the grain structure causes a reduced rate of enrichment of alloying elements at grain boundaries within the workpiece, an improved intergranular diffusion of alloying elements in the workpiece, intergranular diffusion of alloying elements in the area of sensitized metal, and a reduction of substantially straight intergranular paths through the workpiece. Preferably, the workpiece is constructed of a material selected from a group consisting of a carbon steel, a low alloy steel, a high strength steel, a 300-series stainless steel, an aluminum alloy with a magnesium content greater than three weight percent, a copper alloy and a titanium alloy.

According to another aspect of the invention, there is provided a method for treating metal including providing a metal workpiece including a stress corrosion crack, and introducing pulses of ultrasonic wave energy into the workpiece through periodic ultrasonic mechanical impulse impacts. The pulses of ultrasonic wave energy are introduced into a sensitized portion of the workpiece which contains the stress corrosion crack in order to stabilize the metal surrounding the crack for cutting or grinding. After metal stabilization, the section of the workpiece that contains the stress corrosion crack is removed, and a replacement plate is welded within an opening created by the removal of the section. During welding of the plate to the workpiece, additional pulses of ultrasonic wave energy are introduced into the workpiece through additional periodic ultrasonic mechanical impulse impacts that are applied to the root weld and cap weld passes. Instead of removing a section of the workpiece and replacing it with a metal sheet, the crack may be ground out thus leaving a depression in the workpiece. Thereafter, the depression can be filled in with weld metal and treated with additional UIT.

According to yet another aspect of the invention, there is provided a method for treating metal including exposing a metal workpiece to a corrosive environment, wherein the workpiece is susceptible to stress corrosion cracking, and introducing pulses of ultrasonic wave energy into the workpiece through periodic ultrasonic mechanical impulse impacts. The ultrasonic wave energy and periodic ultrasonic mechanical impulse impacts are applied to the workpiece in order to stabilize the workpiece metal thereby making it less susceptible to stress corrosion cracking.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a metallic workpiece surface susceptible to sensitization.

FIG. 2 is a plan view of an metallic workpiece surface that is susceptible to sensitization and exhibiting indeterminate sensitization.

FIG. 3 is a plan view of a sensitized metallic workpiece surface exhibit sensitization.

FIG. 4 is a sectional view of a metallic workpiece that is susceptible to sensitization.

FIG. 5 is a sectional view of a sensitized metallic workpiece that has undergone UIT sensitization remediation in accordance with a preferred embodiment of the present invention.

FIG. 6 is a plan view of a sensitized metallic workpiece including a stress corrosion crack.

FIG. 7 is plan view of the sensitized metallic workpiece of FIG. 6 illustrating a UIT treatment zone about a section of the workpiece containing the stress corrosion crack.

FIG. 8 is a plan view of the sensitized metallic workpiece of FIG. 7 illustrating removal of the section of the workpiece containing the stress corrosion crack.

FIG. 9 is a perspective view of the sensitized metallic workpiece of FIG. 8 illustrating a replacement metal plate welded thereto.

FIG. 10 is a plan view of a sensitized metallic workpiece including a stress corrosion crack.

FIG. 11 is plan view of the sensitized metallic workpiece of FIG. 10 illustrating a UIT treatment zone about the stress corrosion crack.

FIG. 12 is a plan view of the sensitized metallic workpiece of FIG. 11 illustrating removal of the stress corrosion crack.

FIG. 13 is a perspective view of the sensitized metallic workpiece of FIG. 8 illustrating a weld pass along a depression formed by removal of the stress corrosion crack.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the application of UIT to sensitized metals for effectively remediating the effects of metal sensitization and/or repairing the sensitized metals using conventional and emergent welding methods. As used herein, sensitized metal refers to metal having an alloying element precipitate out of solution and congregate at the metal grain boundaries thereby forming a continuous or solid film of the alloying element along the metal grain boundaries. The film may extend to the surface of the metal. By forming a continuous or solid film, interconnected intergranular pathways are formed along the grain boundaries of the metal.

An exemplary metal that is susceptible to sensitization is 5456-H116 aluminum. 5XXX aluminum alloys are commonly used in naval ship structures. These alloys provide high strength-to-weight ratios while maintaining good as-welded strength and excellent corrosion resistance. However, alloys like 5XXX aluminum alloys with above 3 wt % magnesium (Mg) are susceptible to thermal instability. At relatively low temperatures (−70° C.) over varying periods of time from a few years to 10-20 years, the Mg in the aluminum diffuses to grain boundary regions. When the local concentration of Mg is high enough, beta phase (Al₃Mg₂) forms in order to lower the stored energy in the material. The beta phase is anodic to the matrix of alloy in seawater and sea air and this potential difference provides the driving force for dissolution of the beta from the grain boundaries causing intergranular corrosion.

Depicted at FIGS. 1 through 3 are micrographs of 5XXX aluminum workpiece surfaces. FIG. 1 depicts a workpiece surface exhibiting little to no precipitation of beta phase Al₃ Mg₂ along the grain boundaries. In this micrograph, the grain boundaries of the aluminum are visible as disjointed dots across the surface of the workpiece. FIG. 1 is representative of a metal that is not sensitized. FIG. 2 depicts a workpiece surface exhibiting indeterminate aggregating of beta phase Al₃Mg₂ along the metal grain boundaries. The grain boundaries are more defined than in FIG. 1 and are visible as disjointed dots and disjointed, short lines along the metal grain boundaries. This is the result of migration of the beta phase Al₃Mg₂ to the grain boundaries making the boundaries more visible than the boundaries in the 5XXX aluminum workpiece of FIG. 1. However, since the grain boundaries are not visible as continuous or solid lines, indicating only nominal beta phase migration, the 5XXX aluminum workpiece depicted in FIG. 2 is not a sensitized metal. FIG. 3 depicts a workpiece surface exhibiting substantial aggregation of beta phase Al₃Mg₂ along the 5XXX aluminum workpiece grain boundaries. In this instance, the grain boundaries are visible as series of solid, interconnected lines along the metal grain boundaries. These lines represent a film of beta phase Al₃Mg₂ at the boundaries and is indicative of a sensitized metal.

Sensitization of metals is problematic since sensitized metals are susceptible to stress corrosion cracking. Stress corrosion cracking occurs when a material susceptible to stress corrosion, such as a sensitized metal, is exposed to a corrosive environment and tensile stresses are experienced in the material above a threshold value. In a sensitized metal, stress corrosion cracking results from the penetration of corrosive elements of the corrosive environment into the metal along pathways created by intergranular corrosion of the metal along the grain boundaries by the continuous film of precipitated alloying elements. By exposing the internal grain boundary surfaces of the metal to the corrosive elements, the metal is further degraded along the grain boundaries causing further intergranular corrosion and the formation of cracks which are exacerbated by the presence of tensile stresses.

It has been discovered that by treating sensitized metal with UIT, the susceptibility of the metal to stress corrosion cracking can be reduced or eliminated. It has further been discovered that metals exhibiting a stress corrosion crack can be repaired more efficiently than utilizing present methods if the metal undergoes UIT before, during and after the stress corrosion crack is removed. UIT, as used herein and described in detail in U.S. Pat. Nos. 7,431,779; 7,344,609; 7,301,123; 7,276,824; 6,932,876; 6,843,957; 6,289,736, and 6,171,415, all of which are incorporated herein by reference in their entireties, refers to a process of introducing pulse wave energy in combination with ultrasonic mechanical impulse impacts into a load bearing work body's interior structure in such magnitude as to affect or improve the grain structure and the residual stress patterns therein. In particular, the pulse wave energy and impulse impacts cause compressing of the top layer of the metal body and expanding of the top metallic layer in all directions parallel to the metal's surface. The surface layer expands beyond its elastic limit and experiences plasticity, which means that it experiences a permanent tensile strain. The surrounding elastically deformed material opposes this tensile strain thereby imparting compressive residual stresses in the surface of the metal in directions parallel to the surface. By expanding the top layer of the metal, the corrosive elements of the corrosive environment are prevented access to the internal grain boundaries of the metal. Thus, the corrosive elements cannot penetrate the metal. Further, by imparting compressive residual stresses in the metal, the effects of the tensile stresses can be ameliorated.

More particularly, depicted at FIGS. 4 and 5 are sectional views of the grain boundaries of two metal workpieces. In FIG. 4, the represented workpiece has not undergone UIT treatment. In this instance, the crystal grains of the metal have a cuboidal shape or cross-section. The grain boundaries arranged between the cuboidal-shaped crystal grain extend generally vertically and laterally. In a sensitized metal, the vertically-extending grain boundaries present pathways 10 along which stress corrosion cracks can form and exit to the surface of the metal thereby increasing the likelihood of failure of the metal workpiece. In FIG. 5, the represented workpiece has undergone UIT treatment. In this instance, crystal grains near the surface of the workpiece have been expanded in all directions parallel to the workpiece metal surface. The individual metal grains are transformed from a cuboidal shape to a flattened or pancake shape having major axes that extend parallel to the surface of the workpiece surface. By flattening of the metal crystal grains near the surface of the workpiece in a sensitized metal, the intergranular pathways 12 along which stress corrosion cracking can occur become more convoluted and longer than the intergranular pathways 10 in untreated sensitized metals. The result of this grain structure modification is that subsurface defects in the material lack a clear intergranular path to the surface, thus delaying cracks from propagating to a workpiece surface. Grain modification thereby forces a crack to try and propagate across a grain itself, which is a more difficult path requiring more energy to propagate. Further, when the workpiece is located in a corrosive environment, the rate at which the corrosive elements penetrate through the surface of the metal and into the workpiece is reduced.

The penetration of the corrosive elements into the workpiece along pathways 12 may further reduced or altogether eliminated utilizing UIT. Microstructural investigation has shown that UIT treatment of metals produces ultrafine grain structure in the nanocrystalline regimen of the metal down to a depth from the surface of about 6-10 μm. This grain refinement process has been suggested to follow formation of high dislocation density and twining structure following further straining, formation of microbands structure, subdivision of microbands structure into submicron grains, and further breakdown of the subgrains to be equiaxed. Thus, UIT treatment of metals can achieve nanocrystallization of the surface layer for the metal, which is believed to improve in the corrosion and fatigue properties of the materials. In the context of UIT treated sensitized metal, it is believed the nanocrystallization of the surface layer for the metal can likely prevent essentially all penetration by corrosive elements into the workpiece.

In addition to reducing or preventing corrosion element penetration of the metal workpiece and increasing the energy required to propagate a crack within the workpiece, it is believed that UIT treatment of a sensitized metal slows further enrichment of alloying elements at grain boundaries. As explained above, sensitization is a result of enrichment of one or more alloying elements at the grain boundaries. An example are aluminum alloys with magnesium content greater than three weight percent, such as the 5XXX alloy family. The beta phase Al₃Mg₂ rich in magnesium tends to migrate to the grain boundaries. This results in intergranular corrosion and/or greater susceptibility to external, environmental corrosion factors. Accelerated corrosion may occur along a path of higher than normal corrosion susceptibility, which is the along the grain boundaries of a sensitized material where precipitates have migrated to, with the bulk of the material typically being passive. By imparting compressive residual stresses and stress relaxation with UIT, coupled with engineered repairs, the migration of precipitates to the grain boundaries may be slowed to such an extent that stress corrosion cracking no longer effectively influences the service life of the structures. This may be due in part to stabilization of the metal by the introduction of ultrasonic energy, ultrasonic or impulse relaxation, compressive residual stresses or a combination thereof. UIT is also believed to reverse metal sensitization by causing intergranular diffusion of alloying elements thereby eliminating enrichment of alloying elements at grain boundaries and regenerating the metal. To do so, the energy imparted to the base metal by UIT must be of sufficient magnitude to cause the precipitates to return to solution.

To impart the requisite pulse wave energy and ultrasonic mechanical impulse impacts to a metal body to obtain the metal grain and metal grain boundary modifications discussed above, an ultrasonic impact operating system as described in U.S. Pat. No. 6,932,876 can be used. That system employs a set of ultrasonically movable impacting elements, presented typically as sets of three or four spaced members, for impacting a metallic work surface under control of an ultrasonic transducer head. A periodic pulse energy source, typically operable at ultrasonic frequencies up to 100 kHz, induces oscillations into the transducer head, preferably subject to feedback frequency and phase control processing feedback from the working transducer head to aid in matching resonance characteristics of the head when working on the work surface in the manner more particularly set forth in the parent applications of U.S. Pat. No. 6,932,876. The impacting element set creates at the work surface and extending into the sub-surface region of a metallic work body, plasticized metal permitting the surface texture to be machined and sub-surface structural modifications in the work body material to be retained, UIT imparts both ultrasonic relaxation and impulse relaxation within the material. These two components of UIT reduce the magnitude of the tensile residual stresses in the material at greater depths than the plasticity induced compressive stresses which are a surface phenomenon. These methods of relaxation or combinations thereof may result in the resultant tensile stress to be below the threshold value that is a pre-requisite for stress corrosion cracking.

FIGS. 6 though 13 depict two engineering repair methods utilizing UIT on a sensitized metal workpiece including stress corrosion-induced damage. In FIGS. 6 through 9, the damaged workpiece includes a crack and sufficient metal sensitization around the crack that the crack and a portion of the surrounding workpiece metal must be removed and replaced. In FIGS. 10 through 13, the damaged workpiece includes a crack with a level of sensitization of the metal around the crack that only the surface of the metal defining the crack is removed and replaced.

More particularly, referring to FIGS. 6 and 7, there is depicted a sensitized metal workpiece 16, such as a 5XXX aluminum alloy workpiece, including a crack 18 created by stress corrosion. According to this method, a treatment zone 20 is produced in workpiece 16 by introducing pulse wave energy and ultrasonic mechanical impulse impacts to workpiece 16 by utilizing the ultrasonic impact operating system discussed above and thereby modifying the metal as described above. Treatment zone 20 is formed around a section 22 of workpiece that includes crack 18 and other metal that is sufficiently sensitized or otherwise damaged metal to require that the metal be removed from the workpiece. By forming treatment zone 20, the metal therein is stabilized allowing for a cut to be made within the treatment zone and removal of damaged zone 22 from the workpiece without causing additional potentially damaging stresses to the metal.

Referring to FIGS. 8 and 9, following pre-treatment of workpiece 16 with UIT, section 22 and, optionally a portion of treatment zone 20 situated adjacent to section 22, are cut from workpiece 16 thereby forming an opening 24 within workpiece 16 and treatment zone 20. Opening 20 is covered by placing a replacement metal sheet 26 sheet and butt welding sheet 26 within opening 20 along joint 28. Following deposition of the root pass weld along joint 28, UIT is applied along the root pass body and toes to strengthen the weld metal against sensitization and to relax any stress within the metal caused by cutting, fit up and welding. Where multi-pass welds are required, UIT can be applied to the fill passes. A final cap pass UIT application is applied along the cap pass weld of joint 28. Following UIT application along the weld passes, additional UIT is applied to the weld heat affected areas of replacement metal sheet 26 and workpiece 16 adjacent joint 28.

Referring to FIGS. 10 and 11, there is depicted a sensitized metal workpiece 30, such as a 5XXX aluminum alloy workpiece, including a crack 32 created by stress corrosion. According to this method, a treatment zone 34 is produced in workpiece 30 by introducing pulse wave energy and ultrasonic mechanical impulse impacts to workpiece 30 by utilizing the ultrasonic impact operating system discussed above and thereby modifying the metal as described above. Treatment zone 32 is formed around crack 32, including crack 32, and follows the general shape of crack 32. By forming treatment zone 32, the metal therein is stabilized allowing for crack 32 to be removed from the workpiece without causing additional potentially damaging stresses to the metal.

Referring to FIGS. 12 and 13, following pre-treatment of workpiece 30 with UIT, crack 32 is removed from workpiece 16 by grinding thereby forming a depression 36 within workpiece 30 and treatment zone 34. Depression 36 may or may not extend through workpiece 30. Following removal of crack 32, a weld material 38 is deposited within depression 36 thereby filling the depression. Following deposition of the root pass weld within depression 36, UIT is applied along the root pass to strengthen the weld metal against sensitization and to relax any stress within the metal caused by cutting, fit up and welding. A final cap pass UIT application is applied along the cap pass weld of depression 36. As described above, for FIGS. 6 through 9, UIT can be applied to the fill passes when multi-pass welds are required. Further, following UIT application along the weld passes, additional UIT is applied to the weld heat affected areas of replacement metal sheet 26 and workpiece 16 adjacent joint 28.

As will be apparent to one skilled in the art, various modifications can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the claims below. 

It is claimed:
 1. A method for treating metal comprising: providing a workpiece including an area of sensitized metal, and decreasing tensile stresses in the area of sensitized metal by imparting compressive residual stress in the area of sensitized metal.
 2. The method according to claim 1 wherein imparting compressive residual stress in the area of sensitized metal causes one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof.
 3. The method according to claim 1 further comprising applying a multiplicity of shock pulses to the area of sensitized metal thereby creating a treatment zone of plastic material in the metal structure.
 4. The method according to claim 3 further comprising inhibiting, by reaction to the shock pulses in the treatment zone, the migration of alloying element precipitates of the workpiece to grain boundaries in the area of sensitized metal.
 5. The method according to claim 3 further comprising applying the shock pulses in the form of ultrasonic energy with an ultrasonic transducer that is coupled with a surface of the workpiece.
 6. The method according to claim 1 further comprising modifying a grain structure of the area of sensitization by arranging a major axis of each grain of a portion of grains of the grain structure to be essentially parallel to a surface of the area of sensitized metal.
 7. The method according to claim 1 further comprising replacing a damaged portion of the area of sensitized metal with a replacement metal and introducing ultrasound compression wave energy into the workpiece about a junction of the replacement metal with the workpiece.
 8. The method according to claim 1 further comprising contacting an external surface of the area of sensitized metal with a set of striking needles driven by an ultrasonic transducer emitting head.
 9. The method according to claim 1 further comprising modifying the grain structure of the area of sensitized metal by re-orienting a grain structure of the workpiece into a pancake-like structure.
 10. A method a treating metal comprising: providing a workpiece including an area of sensitized metal, the area of sensitized metal including a grain structure including a plurality of crystal grains, and creating a treatment zone in the area of sensitized metal by modifying the grain structure by arranging a major axis of each grain of a portion of grains of the plurality of crystal grains to be essentially parallel to a surface of the area of sensitized metal.
 11. The method according to claim 10 wherein arranging the major axis of each grain of the portion of grains of the plurality of crystal grains to be essentially parallel to the surface of the area of sensitized metal causes one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof.
 12. The method according to claim 10 wherein each grain of the portion of grains of the plurality of crystal grains is arranged to be essentially parallel to a surface of the area of sensitized metal by applying a multiplicity of shock pulses to the area of sensitized metal.
 13. The method according to claim 10 further comprising applying the shock pulses in the form of ultrasonic energy with an ultrasonic transducer that is coupled to a surface of the workpiece.
 14. The method according to claim 10 further comprising removing a damaged portion of the treatment zone from the workpiece and replacing the damaged portion with a replacement metal.
 15. The method according to claim 10 further comprising removing a damaged portion of the treatment zone from the workpiece, replacing the damaged portion with a replacement metal and introducing ultrasonic compression wave energy into the replacement metal.
 16. The method according to claim 10 further comprising contacting an external surface of the area of sensitized metal with a set of striking needles driven by an ultrasonic transducer emitting head.
 17. A method for treating metal comprising: providing a workpiece including an area of sensitized metal, and introducing ultrasound compression wave energy into the area of sensitized metal thereby creating a treatment zone in the area of sensitized metal.
 18. The method according to claim 17 further comprising removing a first portion of the treatment zone from the workpiece.
 19. The method according to claim 18 wherein the first portion of the treatment zone includes a crack.
 20. The method according to claim 18 further comprising welding a replacement metal portion to the workpiece and within a hole formed by removal of the first portion.
 21. The method according to claim 20 further comprising introducing an additional ultrasound compression wave energy into the workpiece about a junction of the replacement metal with the workpiece.
 22. The method according to claim 21 wherein the additional ultrasound compression wave energy is introduced into the workpiece along a root pass weld.
 23. The method according to claim 21 wherein the additional ultrasound compression wave energy is introduced into the workpiece along a cap pass weld.
 24. The method according to claim 18 wherein introducing the ultrasound compression wave energy into the workpiece causes one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof.
 25. The method according to claim 18 further comprising applying a multiplicity of shock pulses to the area of sensitized metal thereby creating the treatment zone of plastic material in the metal structure.
 26. The method according to claim 25 further comprising applying the shock pulses in the form of ultrasonic energy with an ultrasonic transducer that is coupled with a surface of the workpiece.
 27. The method according to claim 25 further comprising modifying a grain structure of the area of sensitization by arranging a major axis of each grain of a portion of grains of the grain structure to be essentially parallel to a surface of the area of sensitized metal.
 28. The method according to claim 18 further comprising contacting an external surface of the area of sensitized metal with a set of striking needles driven by an ultrasonic transducer emitting head.
 29. A workpiece comprising: a sensitized metal portion including a treatment zone, the treatment zone being constructed and arranged by introducing pulses of ultrasonic wave energy into the sensitized metal portion through periodic ultrasonic mechanical impulse impacts.
 30. The workpiece according to claim 29 wherein the treatment zone exhibits one or more of (a) a reduced rate of enrichment of an alloying element at grain boundaries within the area of sensitized metal, (b) an intergranular diffusion of an alloying element in the area of sensitized metal, or (c) a reduction of substantially straight intergranular paths through the grain boundaries of the workpiece to a surface thereof.
 31. The workpiece according to claim 29 wherein the workpiece is constructed of a material selected from a group consisting of a carbon steel, a low alloy steel, a high strength steel, a 300-series stainless steel, an aluminum alloy with a magnesium content greater than three weight percent, a copper alloy and a titanium alloy.
 32. The workpiece according to claim 29 wherein the sensitized metal portion includes a grain structure including a plurality of crystal grains, each grain of a portion of grains of the plurality of grains having a major axis arranged essentially parallel to a surface of the treatment zone.
 33. The workpiece according to claim 29 further comprising an opening extending through the treatment zone created by removal of a damaged section of the sensitized metal portion.
 34. The workpiece according to claim 29 further comprising a metallic replacement member welded to the workpiece within an opening through the treatment zone created by removal of a damaged section of the sensitized metal portion.
 35. The workpiece according to claim 29 further comprising a stress corrosion crack in the treatment zone.
 36. The workpiece according to claim 29 further comprising a depression formed within the treatment zone, the depression being created by removal of a crack, wherein the depression is filled with a replacement metal.
 37. A method for treating metal comprising: providing a metal workpiece including a stress corrosion crack, and introducing pulses of ultrasonic wave energy into the workpiece through periodic ultrasonic mechanical impulse impacts.
 38. The method according to claim 37 wherein the pulses of ultrasonic wave energy are introduced into a sensitized portion of the workpiece which contains the stress corrosion crack.
 39. The method according to claim 37 further comprising removing a section of the workpiece that contains the stress corrosion crack and welding a replacement plate within an opening created by the removal of the section.
 40. The method according to claim 39 further comprising introducing additional pulses of ultrasonic wave energy into the workpiece through additional periodic ultrasonic mechanical impulse impacts wherein the additional periodic ultrasonic mechanical impulse impacts are applied to a weld joint formed between the workpiece and the replacement plate.
 41. A method for treating metal comprising: exposing a metal workpiece to a corrosive environment, wherein the workpiece is susceptible to stress corrosion cracking, and introducing pulses of ultrasonic wave energy into the workpiece through periodic ultrasonic mechanical impulse impacts.
 42. The method according to claim 41 wherein the pulses of ultrasonic wave energy are introduced into a sensitized portion of the workpiece which contains a stress corrosion crack. 